Underground Investigation and Risk Management Part 2: Risk and Projects
In the last newsletter, we covered “What is Risk?”. In this second article, we will discuss how risk and projects interact.
There are many complex models and references for geotechnical risk management, from the beginning (conceptual design stage) to end of the project (construction and operation). A proper geotechnical risk management plan has many detailed steps involved in it, such as identifying the hazards, sharing risk, etc. Although a geotechnical investigation is just one step in the process, it is by far the most important one.
A geotechnical investigation done improperly can cause significant construction delays, increased cost overruns, redesigns, and much more, ultimately and negatively impacting a project. A recent study of 41 legal court cases, where the claim was related to geotechnical issues, revealed that 55% of the claims were the result of “changed soil conditions.” This type of claim is directly related to the geotechnical investigation. Other types of claims such as “design inaccuracies” may also be rooted in the geotechnical investigation. This same study also found that approximately 45% of the claims against consultants were directly related to the geotechnical investigation.
What can be done to bring these numbers down? Conduct more borehole tests? Or maybe it just makes more sense to do an effective site investigation in the first place. Geotechnical unknowns, hence uncertainties, usually exist in reverse correlation to the scale and quality of the investigation. We are always asked by the project managers, “What is the reduction in risk for each dollar I spend on a geotechnical investigation?”. Unfortunately, the correlation between the scale of geotechnical investigation (i.e. the number of boreholes) and risk is not a universal one. Different geological regions have different soil deposits/property variability. Even for a site or region with natural variations of geotechnical conditions, different construction methods have different sensitivity to the geotechnical condition. Risk is the effect of the threats, not the threat itself. Therefore, the project affects a difference on the risk level and hence a difference on the required level of geotechnical investigation. For example, an underground tunnel excavation is more sensitive than an open trench installation. Subsequently, one conducts more boreholes for a tunnel as compared to an open trench excavation.
Now that you have received the crushing blow telling you that no one geotechnical investigation can function universally, here is the good news – It is possible to arrive at an upper bound for necessary geotechnical work!
This idea started by studying the cost overruns in tunnelling projects. The study shows an upper bound correlation between the ratio of total length of boreholes over tunnel length as compared to the cost overruns. The common median ratio of Borehole: Tunnel Lengths 0.42 and there are tunnel projects with 50% or more cost overruns when the scale of geotechnical investigation is this low and at this median BH: Tunnel ratio. The same data suggests that if the number of the boreholes are increased to a ratio of 1 (BH: Tunnel), the upper estimate of possible cost overrun can be reduced to a more reasonable estimate of 15-20%. The study also suggests, there is not much benefit going beyond a BH:Tunnel ratio of 1.5. Several graphs are summarizing this study; two of them are provided below. The graph on above right shows the contractors bid as a percentage of as-completed construction cost. When the scale of geotechnical investigation is low, the contractors have overestimated the bid (money out of owners/taxpayer pocket) by 30% or underestimated the bid (future claim) by 30% to 40%. The reality is, the more information a contractor has about subsurface geotechnical conditions, the more informed and competitive the contractor’s bid becomes.
Generally, we prefer to have this type of information in the context of the investigation cost. For major infrastructure projects, it is recommended to budget, for all phases of the geotechnical investigation, costs ranging from 1.5% to 2.2% of the construction cost and have a contingency up to 3.0% of construction cost (for complex condition). For low-rise buildings, that number is as low as 0.2 to 0.4% of construction cost. It is safe to say all the construction projects generally sit somewhere between a low-rise building and a major infrastructure project.
Cost of an investigation is not always an accurate indication of its effectiveness. Appropriate choice of investigation methods, research on the project area (other construction, existing geotechnical information), geological modelling, quality of the investigation, and a proper engineering assessment all play a key role in conducting a truly effective investigation. By employing these principles, it is possible to greatly reduce the geotechnical risk while spending the same level of money on the investigation. Don’t just spend money, do the right investigation!
Stay tuned for the 3rd and final installment of this article series by Masoud Manzari in our next newsletter, where he will bring it all home to discuss how you can manage geotechnical risk the right way.
Masoud Manzari is a senior geotechnical and hydrogeological engineer with 23+ years of experience and has been involved in a wide range of civil engineering projects, specifically for structures built on challenging soil sites. He has also worked as a University Lecturer during his doctorate study. Masoud has worked on many different phases of projects, including site investigation, analysis and design, construction, forensic engineering and remedial measures. He is the past chair of the Canadian Geotechnical Society-Southern Ontario Section and the geotechnical lead for the design of the Eglinton Light Rail Transit since 2010.